Railroad grade crossings (sometimes referred to in the U.K. as level crossings) are locations at which railroad tracks intersect roads. Avoiding collisions between people, trains and automobiles at grade crossings has always been a matter of great concern in the railroad industry.
Warning systems have been developed to warn people and cars of an approaching train at a grade crossing. These warning systems typically include lights, bells and one or more gate arms (e.g., the familiar black and white striped wooden or fiberglass arms often found at highway grade crossings) that block the road and/or sidewalks when a train is approaching the crossing. The lights, bells and gate arms of these warning systems are typically controlled by a controller. Most controllers in use in the U.S. today utilize an input from a grade crossing predictor circuit to determine when to activate the warning system. A crossing predictor circuit is an electronic device which is connected to the rails of a railroad track and is configured to detect the presence of an approaching train, determine its speed and distance from a crossing, and use this information to generate a constant warning time signal for control of a crossing warning device. Other techniques for providing an input to a controller include laser-based systems for detecting a train and determining its distance and speed.
These known systems share a common characteristic: they are independent of any active signal from a train. In other words, these systems detect a train but do not rely on the train to generate any control signals.
Another characteristic of these known systems is that, although they are highly reliable, they are not perfect and have been known to malfunction on occasion. Such a malfunction can take the form of a warning system activating (e.g., a gate staying in a lowered position) when no train is approaching and, more dangerously, a warning system failing to activate (e.g., a gate staying in the raised position) when a train is approaching.
A more recent development in train safety has been the use of positive train control, or PTC, systems onboard locomotives. These systems are designed to prevent collisions between trains, to enforce speed restrictions, and to perform other safety-related functions. Although these systems vary widely in their implementation, many of them share common characteristics such as a positioning systems and map databases that allow a locomotive to determine its position relative to a track system and communications system that allow the locomotive to communicate with devices located off of the train.
It is known in the art to utilize such locomotive PTC systems as a means to ensure that a train does not pass a grade crossing when a warning system is malfunctioning. The leading patent in this area is U.S. Pat. No. 6,996,461 to Kane et al. In Kane's system, a train approaching a grade crossing transmits an interrogation signal to a wayside device such as a grade crossing controller prior to reaching the grade crossing, and does not go through the crossing if a response indicating that the warning system has been properly activated has been received. Note that Kane's system does not trigger activation of the crossing warning system or control it in any way; rather, Kane's system only interrogates the wayside warning system to determine if it has activated prior to the train passing the crossing.
Another system, described in U.S. Pat. No. 5,620,155 to Michalek, discloses an system located onboard a locomotive that can send a signal to a wayside warning system to activate the wayside warning system. Michalek's system, however, operates by sending an activation signal to the warning system when the train is at a predetermined distance from the crossing. This is wasteful as such a scheme will cause the warning system to activate in advance of when necessary for a slow moving train (it being understood that the predetermined distance must be sufficiently spaced apart from the crossing to allow for a train traveling at the highest allowable speed). This drawback might be tolerable for rural crossings with warning devices consisting of only flashing lights as cars may be able to pull up to the tracks, determine the distance of the train, and proceed through the crossing if the train is still far away (although this is still wasteful as the car is forced to slow down or stop needlessly). However, such a system is far less tolerable for crossings with gates that prevent cars from going through the crossing when the warning system is active.